Hydrodynamic Reduction Research Articles

Preliminary Analysis of Hydrodynamic Drag Reduction and Fouling Resistance of Surfaces Inspired by the Mollusk Shell, Dosinia juvenilis.

Many species of plants and animals show an ability to resist fouling with surface topographies tailored to their environments. The mollusk species Dosinia juvenilis has demonstrated the ability to resist the accumulation of fouling on its outer surface. Understanding the functional mechanism employed by nature represents a significant opportunity for the persistent challenges of many industrial and consumer applications. Using a biomimetic approach, this study investigates the underlying hydrodynamic mechanisms of fouling resistance through Large Eddy simulations of a turbulent boundary layer above a novel ribletted surface topography bio-inspired by the Dosinia juvenilis. The results indicate a maximum drag reduction of 6.8% relative to a flat surface. The flow statistics near the surface are analogous to those observed for other ribletted surfaces in that the appropriately sized riblets effectively reduce the spanwise and wall-normal velocity fluctuations near the surface. This study supports the understanding that nature employs ribletted surfaces toward multiple functionalities including the considered drag reduction and fouling resistance.

Enhancing hydrodynamic drag and lift reduction around two staggered obstacles via control plates

This study analyzes the enhancement of hydrodynamic forces reduction for flow around two identical square cylinders arranged in a staggered manner utilizing the control plates. Effects of attached flat plates on flow structures and fluid forces acting on cylinders are examined by considering plates length (l) = 0.1–10 times cylinder’s size. The gap distance between cylinders (g) is fixed at 2.5 times cylinders' size with a fixed Reynolds number of 200. Four different flow patterns revealed in this study: synchronized flow pattern for l = 0.1 to 0.6, quasi flip-flopping flow pattern for l = 0.7 to 1.9, modulated synchronized flow pattern for l = 2 to 4.1 and quasi-periodic flow pattern for l = 4.2 to 10 while non-synchronized flow pattern is observed for staggered cylinders without plates at g = 2.5. The flow patterns variation is found to be directly linked with variations in Strouhal number (St) of both cylinders. The fluid force parameters decreased significantly with increasing length of plates as compared to two staggered cylinders without plates. In particular, mean drag coefficient (CDmean) reduces maximum up to 91% at l = 0.8, root-mean-square value of drag coefficient (CDrms) reduces up to 82% at l = 0.1, root-mean-square value of lift coefficient (CLrms) reduces up to 94% at l = 0.3, St reduces up to 72% at l = 10, the amplitude of drag coefficient (CDamp) reduces up to 78% at l = 0.8 and amplitude of lift coefficient (CLamp) reduces up to 87% at l = 0.2. The plates are found more effective in controlling the flow around the second cylinder as compared to the first cylinder. This study also reveals that at some plate lengths, instead of reduction, an increase in hydrodynamic force values occurred compared to the uncontrolled case.

Hydrodynamic drag reduction in ribbed microchannel with infused non-Newtonian lubricants

Liquid-infused surfaces have recently gained prominence in engineering applications owing to their versatile characteristics such as self-cleaning, anti-fogging, drag reduction, and enhanced heat transfer. In this article, a numerical analysis of pressure-driven flow past a periodic array of rectangular transverse grooves infused with non-Newtonian immiscible lubricants is performed. The volume of fluid method is employed to capture the interface between primary and secondary fluids, and the power-law model is deployed to mimic the non-Newtonian lubricant. The drag reduction capability of the microchannel is examined for various parameters such as Reynolds number, liquid fraction, viscosity ratio, viscosity index, and contact angle. It is observed that the introduction of a non-Newtonian fluid (shear-thickening or shear-thinning) drastically modifies the interface velocity and hydrodynamic resistance. In particular, a shear-thinning lubricant enhances the slip length as the viscosity index (n) is reduced owing to the reduced viscosity at the interface. Note that, for a lubricant having n = 0.7, the percentage improvement in the slip length is 382% in comparison with a Newtonian counterpart having the same viscosity ratio, N = 0.1. Importantly, the introduction of a shear-thinning lubricant with a viscosity ratio N = 5, a liquid fraction of 0.8, and a behavior index n = 0.7 yielded a pressure drag reduction of 63.6% with respect to a classical no-slip channel and of 23% with reference to a microchannel with the Newtonian lubricant. Moreover, at high Reynolds numbers, Re→50, the drag mitigation is slightly lowered due to the primary vortex shift in the cavity. Furthermore, the effect of the interface contact angle (θc) is investigated, as θc drops from 90° (flat) to 45° (convex); the meniscus curvature is enhanced, and the effective slip length is reduced. These observations suggest that a shear-thinning lubricant-infused microchannel is a promising candidate for drag reduction in lab-on-chip applications.

Study on Hydrodynamic Lubrication and Friction Reduction Performance of Cylindrical Rolls With Tesla Valve Texture

Friction and wear seriously affect the efficiency and life of a machine device. The dynamic pressure lubrication and friction properties of the surface are improved by surface texture technology, which plays an extremely important role in reducing friction and wear. In order to maximize the hydrodynamic lubrication and friction reduction performance of surface texture, a Tesla valve surface texture is proposed in this article. The oil film bearing capacity and wall friction of Tesla valve texture are examined through computational fluid dynamics fluid simulation. Cylindrical rolls with Tesla valve texture are prepared by using laser processing technology, and comparative friction wear tests are conducted. The results show that the lubrication performance of Tesla valve texture is four times higher than without this texture. The friction coefficient and wear loss of the cylindrical rolls with Tesla valve texture are, respectively, 39.7% and 66.7% lower than those of the cylindrical rolls without texture. Tesla valve texture effectively improves the dynamic pressure lubrication and friction in the cylindrical rolls.

Sustainability of the plastron on nano-grass-covered micro-trench superhydrophobic surfaces in high-speed flows of open water

This paper studies the sustainability of plastrons on superhydrophobic (SHPo) surfaces made of longitudinal micro-trenches covered by nano-grass with the main interest on hydrodynamic friction drag reduction in high-speed flows of open water, which represent the operating conditions of common watercraft. After revising the shear-driven drainage model to address the air diffusion for SHPo surfaces, the existing theories are combined to reveal the trends of how the immersion depth, air saturation level and shear stress affect the maximum attainable plastron length. Deviations from the theories by the dynamic effect at the two ends of the trench, the interfacial contaminations and turbulent fluctuation are also discussed. A combinatorial series of well-defined SHPo trench surfaces (4 cm × 7 cm in size with varying trench widths, depths, lengths and roughnesses) is microfabricated and attached underneath a 4 m long motorboat on seawater in turbulent flows up to 7.2 m s−1(shear rate ∼83 000 s−1and friction Reynolds number ∼5500). Because the plastron can provide a substantial slip only while its air–water interfaces are pinned (or only slightly depinned) at the trench top, two underwater cameras are employed to differentiate the pinned (and slightly depinned) interfaces from the depinned (and no) interfaces. In addition to achieving pinned plastrons on 6 cm long trenches aligned to high-speed flows in open water, the experimental results corroborate the theoretical estimations, supporting the design of SHPo surfaces for field applications.

Effect of Fouling on the Performance of an Instream Turbine

As the tidal energy industry starts to mature towards commercial projects a key focus is on reliable power performance. As for any marine application, fouling poses a potential performance reduction risk for instream turbine deployments.
 SCHOTTEL HYDRO have developed their current commercial SCHOTTEL Instream Turbines. Four drivetrains with 6.3m rotors were deployed on the surface platform PLAT-I by Sustainable Marine Energy. One of PLAT-Is key features is access to the turbines for inspection and maintenance in situ. The system has undergone sea testing from 2017 to 2021 in Scotland and Nova Scotia (Canada).
 This paper presents the hydrodynamic rotor performance reduction due to fouling based on full-scale experimental results. An in-house blade element momentum model is used to quantify the changes of the hydrodynamic forces in terms of lift and drag for the hydrofoils used. Furthermore, the effect of fouling on the downstream wake was quantified in the field. The performance reduction due to fouling is significant and leads to a power drop of up to 43%, whereas the thrust is reduced by 25%. This is also reflected in a reduction of the turbine’s downstream wake as a “fouled” rotor extracts less energy from the flow. Modifications of the polar data, used for semi-empirical performance predictions, are able to predict the effect of fouling on the rotor performance.
 In general, the results derived from the testing prove the significance of access to the turbines in order to avoid reduction in the turbines’ performance due to fouling.

Hydrodynamic cavitation reduction in semidilute turbulent polymer solution flows

The hydrodynamic cavitation in semidilute solution flows of a flexible polymer additive in water was experimentally explored in a mesoscale converging–diverging nozzle to elucidate the cavitation reduction effects of polymer additives. Rheological measurements demonstrated that polymer solutions were shear-thinning, with infinite viscosities larger than pure water. The polymer additives significantly mitigated the intensity of cloud cavitation and the growth of violent cavity structures in the tested solution concentrations. Under conditions of supercavitation, the tested polymer solutions could not suppress the growth of large structures but showed a reduction in the population of cavitation bubbles. The temporal evolution and spatial variation of cavitation structures in different concentrations were captured using high-speed imaging. Statistical analysis of the images showed that polymers reduce cavitation via three main mechanisms. (1) The longitudinal expansion of cavities downstream is attenuated relative to the pure water. The streamwise distribution of vapour-ratio fluctuations was flattened, and its peak was shifted upstream in the solutions. (2) Mean collapse and growth rate of cavitating bubble pockets and their fluctuations were noticeably relaxed by polymer additives. For a 400 p.p.m. solution (parts per million (p.p.m.)), a reduction of 65 % was measured relative to pure water flow at the highest tested flow rate. (3) Spectral analysis of the downstream pressure indicated that the shedding frequency at the cavitation inception was reduced as the solution's concentration increased. This reduction was as high as 70 % for a 400 p.p.m. solution. These results highlight the strong interplay between polymer additives and the generation of cavitation-related structures.

Study on hydrodynamic lubrication and friction reduction performance of spur gear with groove texture

Spur gears are widely used in aerospace, energy power and other fields due to their advantages of convenient design and manufacturing. However, in the process of spur gear transmission, the tooth surface wear and adhesion are prone to failure, which seriously affect the life of the gear transmission. In order to minimize the frictional wear during gear transmission, a groove-type tooth surface texture is proposed in this paper. The core size parameters of the groove texture are optimized through CFD (Computational Fluid Dynamics) fluid simulation and groove texture cylindrical roller process experiments. Gears with groove texture are prepared by using the laser processing technology, and comparative friction wear tests are conducted. The experimental results show that the maximum and minimum damage area rates of the spur gears with groove texture are, respectively, 84.8% and 84.9% lower than those of the spur gears without groove texture. The anti-adhesion ability is also significantly improved in the gears with grove texture. In addition, the average friction coefficient of the spur gears with groove texture is significantly reduced, which effectively reduces the transmission dynamic pressure lubrication and friction in the spur gears.

Fabrication of Salvinia-inspired surfaces for hydrodynamic drag reduction by capillary-force-induced clustering

For decades, bioinspired functional materials have been attracting the interest of many researchers for their remarkable characteristics. In particular, some plant leaves are well known for their inherent superhydrophobic nature. Salvinia molesta, a free-floating aquatic fern, has egg-beater-shaped hierarchical trichomes on its surface of leaves. Due to the unique structure and complex wettability of the hairs, this plant has the ability to maintain a stable thick air layer upon the structure when it is submerged underwater. Often referred to as the “Salvinia Effect,” this property is expected to be suitable for use in hydrodynamic drag reduction. However, due to the complex shape of the trichome, currently applied fabrication methods are using a three-dimensional printing system, which is not applicable to mass production because of its severely limited productivity. In this work, artificial Salvinia leaf inspired by S. molesta was fabricated using a conventional soft lithography method assisted with capillary-force-induced clustering of micropillar array. The fabrication method suggested in this work proposes a promising strategy for the manufacturing of Salvinia-inspired hydrodynamic drag reduction surfaces.

Seismic Analysis of Baffle-Reinforced Elevated Storage Tank Using Finite Element Method

The sloshing phenomenon is an important field of fluid dynamics in liquid storage tanks under earthquake excitation. When the sloshing frequency gets close to the liquid tank’s natural frequency, the resulting resonance could lead to instability and even damage to structures, followed by catastrophic economic losses and environmental damages. As passive control devices, baffles are a place for liquid energy dissipation. This study uses annular and horizontal baffles to evaluate the baffles’ relative effectiveness on the elevated storage tanks’ dynamic response. The analysis results are compared with those of elevated storage tanks with no baffles. The flexible and rigid storage tank analysis is examined here, where half of the tank height is filled with liquid. The structural interaction between the liquid, the (horizontal and annular) baffle, and the elevated storage tank affected by seismic action are investigated using Abaqus software. The results confirm that using the baffles, the maximum base shear force in flexible and rigid elevated storage tanks decreases as much as 26.43% and 31.90%, respectively, and the maximum hydrodynamic pressure reduction in the tank is 50.1%.

Mechanical Degradation of Biopolymers for Enhanced Oil Recovery Applications

Summary Polymer degradation is detrimental to enhanced oil recovery (EOR) because it ultimately leads to solution viscosity loss. Molecular breakage during flow owing to high stresses, known as mechanical degradation, can occur in numerous stages during field polymer injection. One of the reasons biopolymers are promising for EOR is their enhanced mechanical stability compared to their polyacrylamide counterparts. This study presents a comparative investigation of the mechanical degradation of biopolymers with potential applications in EOR. The mechanical degradation was evaluated by flowing the solutions through a short-length capillary (internal diameter of 0.127 mm and length of 10 mm) and then testing their viscosity loss through a rheometer. Nine flow velocities were tested between 0.13 and 66 m/s (shear rates between 8.29 × 103 and 1.66 × 107 s−1) in addition to the undegraded baseline. Four biopolymers were evaluated [xanthan gum (XG), scleroglucan (SCLG), schizophyllan (SCP), and guar gum (GG)] and compared to two polyacrylamides [hydrolyzed polyacrylamide (HPAM) and HPAM-AMPS (2-acrylamido-2-methylpropane sulfonic acid)] that serve as benchmarks for EOR processes. All the polymers were evaluated in three different concentrations (100, 500, and 2,000 ppm) in synthetic seawater (3.01% total dissolved solids). The degradation was evaluated through the Ostwald-de Waele indices. The consistency index (K) indicates loss of overall viscosity, and the behavior index (n) accounts for the loss of pseudoplasticity. Overall, the mechanical degradation correlated positively with shear rate and negatively with polymer concentration. When increasing the polymer concentration, the dominant mechanisms at play were macromolecule extension inhibition, caused by interactions with surrounding molecules, and hydrodynamic volume reduction. The biopolymers displayed superior resistance to mechanical degradation than the synthetic ones, with XG being the most resistant polymer. In this work, the main factors for the mechanical degradation resistance of different polymers were rigidity of the polymer structure in solution and the molar mass of the polymer chains.

Diffusion and Superdiffusion from Hydrodynamic Projections

Hydrodynamic projections, the projection onto conserved charges representing ballistic propagation of fluid waves, give exact transport results in many-body systems, such as the exact Drude weights. Focussing one one-dimensional systems, I show that this principle can be extended beyond the Euler scale, in particular to the diffusive and superdiffusive scales. By hydrodynamic reduction, Hilbert spaces of observables are constructed that generalise the standard space of conserved densities and describe the finer scales of hydrodynamics. The Green–Kubo formula for the Onsager matrix has a natural expression within the diffusive space. This space is associated with quadratically extensive charges, and projections onto any such charge give generic lower bounds for diffusion. In particular, bilinear expressions in linearly extensive charges lead to explicit diffusion lower bounds calculable from the thermodynamics, and applicable for instance to generic momentum-conserving one-dimensional systems. Bilinear charges are interpreted as covariant derivatives on the manifold of maximal entropy states, and represent the contribution to diffusion from scattering of ballistic waves. An analysis of fractionally extensive charges, combined with clustering properties from the superdiffusion phenomenology, gives lower bounds for superdiffusion exponents. These bounds reproduce the predictions of nonlinear fluctuating hydrodynamics, including the Kardar–Parisi–Zhang exponent 2/3 for sound-like modes, the Levy-distribution exponent 3/5 for heat-like modes, and the full Fibonacci sequence.

Ковариантный формализм гидродинамики гамильтоновых систем

The theory of hydrodynamic reduction of non-autonomous Hamiltonian mechanics (V. Kozlov, 1983) is presented in the geometric formalism of bundles over the time axis R. In this formalism, time is one of the coordinates, not a parameter; the connections describe reference frames and velocity fields of mechanical systems. The equations of the theory are presented in a form that is invariant with respect to time-dependent coordinate transformations and the choice of reference frames.

Investigation on the damping of rectangular water tank with bottom-mounted vertical baffles: Hydrodynamic interaction and frequency reduction effect

Internal obstructions are commonly employed in Tuned Liquid Damper (TLD) tanks to produce sufficient inherent damping for improving its vibration control performance. However, the hydrodynamic interaction between obstacles has a significant effect on damping ratio of liquid sloshing when the obstacles are becoming close to each other. At the same time, the natural sloshing frequency of the water tank decreases because of the added mass produced by these obstacles. By adopting potential flow and linear wave theory, a linearized analysis model is proposed in this paper to estimate the damping ratio of rectangular water tank with bottom-mounted vertical baffles. The formula of its damping ratio is then further modified to take into account the effect of hydrodynamic interaction between vertical baffles on the fluid velocity and the added mass coefficient. In this study, reduction in the natural frequency of liquid sloshing due to vertical baffles is also investigated in detail. Shaking table tests were conducted on scaled models of the rectangular tank with bottom-mounted vertical baffles to validate the accuracy of the proposed analytical model in this study. Parametric studies are performed to study the effect of the distance between vertical baffles, different water depth ratios, various sinusoidal excitation amplitudes, and the different number of bottom-mounted vertical baffles on the damping ratio. The analyzed results showed that good agreement between the proposed analytical solutions and the experimental results was found. The study also showed that the proposed methodology can effectively estimate the damping ratio by considering the hydrodynamic interaction and natural frequency reduction effect due to the installed bottom mounted vertical baffles.

Hydrodynamic drag reduction of shear-thinning liquids in superhydrophobic textured microchannels

Super-hydrophobic textured surfaces reduce hydrodynamic drag in pressure-driven laminar flows in micro-channels. However, despite the wide usage of non-Newtonian liquids in microfluidic devices, the flow behaviour of such liquids was rarely examined so far in the context of friction reduction in textured super-hydrophobic micro-channels. Thus, we have investigated the influence of topologically different rough surfaces on friction reduction of shear-thinning liquids in micro-channels. First, the friction factor ratio (a ratio of friction factor on a textured surface to a plain surface) on generic surface textures, such as posts, holes, longitudinal and transverse ribs, was estimated numerically over a range of Carreau number as a function of microchannel constriction ratio, gas fraction and power-law exponent. Resembling the flow behaviour of Newtonian liquids, the longitudinal ribs and posts have exhibited significantly less flow friction than the transverse ribs and holes while the friction factor ratios of all textures has exhibited non-monotonic variation with the Carreau number. While the minima of the friction factor ratio were noticed at a constant Carreau number irrespective of the microchannel constriction ratio, the minima have shifted to a higher Carreau number with an increase in the power-law index and gas fraction. Experiments were also conducted with aqueous Xanthan Gum liquids in micro-channels. The flow enhancement (the flow rate with super-hydrophobic textures with respect to a smooth surface) exhibited a non-monotonic behaviour and attenuated with an increase in power-law index tantamount to simulations. The results will serve as a guide to design frictionless micro-channels when employing non-Newtonian liquids.

Hydrodynamic instability and flow reduction in polymer brush coated channels.

A polymer brush is a passive medium. At equilibrium the knowledge of its chemical composition and thickness is enough for a full system characterization. However, when the brush is exposed to fluid flow it reveals a much more intriguing nature, in which filamentous protrusions and the way they interact among themselves and with the surrounding fluid are of outmost importance. Here we investigate such a rich behavior via numerical simulations. We focus on the brush hydrodynamic response at low Reynolds numbers, observing a significant fluid flow reduction inside a polymer-brush coated channel. We find that the reduction of the flow inside the channel is significantly larger than what would happen if the brush effect consisted only in reducing the effective channel width. This amplified reduction is understood as being due to the morphological instability of the brush-liquid interface which is shown to have an elastic origin: the mechanical stress acting on the brush due to the imposed flow is partially released by the interface modulation. In turn, this modulation dissipates more energy than a flat interface in the surrounding fluid, causing a reduction of flow velocity. Our results and interpretations provide an explanation for recent experimental measurements.

Thermal behavior of wormlike micelles under turbulent and quiescent regimes

Under turbulent flow, wormlike micelles efficiently cause hydrodynamic drag reduction. However, it was demonstrated that beyond specific temperatures, the capacity of the micellar aggregates to produce drag reduction is abruptly lost. This can be associated with the decrease in size of long micellar chains, becoming shorter aggregates, in the sense that small aggregates are not capable to cause hydrodynamic drag reduction anymore. In this paper, we investigate the thermal effect on wormlike micelles under turbulent and quiescent regimes, in order to explore correlations between the size of the wormlike micelles and the hydrodynamic drag reduction phenomenon.

Wire mask assisted rolling as a cost-effective method for high-throughput surface micro-texturing

Micro-dimples are a specific class of surface textures that are ubiquitous in a wide range of applications where functionalities such as wetting, hydrodynamic drag reduction, and sliding friction reduction are needed. However, fabrication of the micro-dimples through the existing manufacturing techniques involves the use of complex equipment and requires relatively long cycle times. Consequently, we report here proof-of-concept of a method that enables in fabricating micro-dimples in a facile and fast manner. The industrial sieve wire masks were employed to deform metallic sheets at discrete locations by the cold rolling process. The geometry of projections on the wire masks allowed the formation of ellipsoid shaped micro-dimples in two distinct patterns on metallic sheets. The finite element modelling of process-material interactions was accurately predicted the formation of micro-dimples under different reduction ratios. We also demonstrate that the micro-dimples can be easily replicated on polymer surfaces using textured metallic moulds, which facilitates low-cost mass production in microfluidic applications. The presence of micro-dimples on polymer surfaces has led to an increase in water contact angles and resulted in a substantial drag reduction in microchannels. From the tribological viewpoint, the obtained patterns and surface area fractions of the micro-dimples are attractive. These findings provide a promising way to create textured surfaces in an economical manner, with functionalities suitable for microfluidic and tribological devices.

Finite amplitude torsional oscillations of shape-morphing plates immersed in viscous fluids

In this paper, we study torsional oscillations of a cross section of a thin plate submerged in a quiescent, Newtonian, incompressible, and viscous fluid. The plate is subjected to a prescribed shape-morphing deformation in phase with the rigid oscillation. The problem is completely described by three nondimensional parameters indicating oscillation frequency and amplitude and intensity of the shape-morphing deformation. We conduct a parametric study to investigate the possibility of controlling hydrodynamic moments and power dissipation through an active time-varying shape-morphing strategy. The problem is studied in both the linear and nonlinear flow regimes, by employing the boundary element method and direct numerical simulations via computational fluid dynamics methods, respectively. Investigation of flow physics demonstrates that, similarly to what is observed for the case of flexural oscillations, the shape-morphing strategy is effective in modulating vortex shedding in torsional oscillations. The results show that hydrodynamic power dissipation can be minimized and hydrodynamic moments can be controlled through an optimal imposed shape-morphing deformation. Findings from this study are directly applicable to torsional oscillation-based underwater energy harvesting or sensing and actuation systems, where control of hydrodynamic moments and reduction of hydrodynamic power losses are necessary for optimal device operation.

Superhydrophobic Drag Reduction for Turbulent Flows in Open Water

Trapping air under water, superhydrophobic surfaces on boats have long been coveted for hydrodynamic drag reduction without bubble injection. Despite many reports of drag reduction in lab tests, such surfaces have not been successful in real-world tests on open water, at high Reynolds numbers. The authors develop hydrophobic microtrenches that provide a large slip, with a subtle detail that maximizes air retention. Replacing a portion of a motorboat's hull with the developed surface, they obtain large drag reduction under high-speed flows on the open sea. This has significant implications for energy efficiency in oceanic shipping.